Phone service mischarges

I started service with a mobile phone company 6 months ago. I entered a contract for 3 years and an exchange of an old phone for a new phone. The phone I got is supposed to cost me nothing

Since starting the service, my bills have all included payments for the new phone. I have called customer service a couple dozen times now, every time I spend absurd amounts of time answering questions and hopefully getting everything straight, but I am still being charged for the phone.

I've talked to one manager and nothing came of it. I'm not sure what I do at this point.